The invention relates to a process for preparing acrolein from glycerol using an acidic catalyst which comprises tungsten compounds and at least one further promoter.
Acrolein is an important intermediate and is of great economic significance for the preparation of acrylic acid, D,L-methionine and the methionine hydroxy analogue 2-hydroxy-4-methylthiobutyric acid (MHA). Methionine is an essential amino acid which is used, inter alia, as a supplement in feeds. Nutrition-improving feed additives are nowadays an indispensable constituent in animal nutrition. They serve for better utilization of the food supply, stimulate growth and promote protein formation. One of the most important of these additives is the essential amino acid methionine, which assumes a prominent position as a feed additive in poultry breeding in particular. In this field, though, methionine replacements such as methionine hydroxy analogue (abbreviated to MHA) also have not inconsiderable significance, since they have similar growth-stimulating properties to the amino acid known for this purpose. Acrylic acid is an important starting material for preparing polymers which, for example owing to their water absorption capacity, are used as superabsorbents.
According to the prior art, acrolein is synthesized by heterogeneously catalysed selective oxidation of propene over mixed oxide catalysts. EP 417723 describes the synthesis over complex mixed multimetal oxide catalysts at temperatures of 300 to 380° C. and pressures of 1.4 to 2.2 bar. Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 1999 describes the overall process including workup, in which several by-products are removed. Once the reactant mixture of propene, air and water has been converted at least partly over the catalyst, quenching is first effected to remove high-boiling by-products such as polymers, acrylic acid and acetic acid. In the downstream absorber, acrolein is washed out. After the desorption, the absorbent is recovered by purifying the crude acrolein obtained by distillation in several stages.
It is known that glycerol can be dehydrated in the presence of acidic substances to various products. According to Organic Synthesis I, 15-18 (1964), treatment of a mixture of pulverulent potassium hydrogensulphate, potassium sulphate and glycerol at 190 to 200° C. affords acrolein in a yield of between 33 and 48%. Owing to the low yields and the high salt burdens, this process is, however, not suitable for the industrial scale.
In the course of studies of model substances of biomass pyrolysis oils, the catalytic treatment of glycerol over H-ZSM5 zeolites at 350 to 500° C. has also been studied—see Dao, Le H. et al. ACS Symp. Ser.: 376 (Pyrolysis Oils Biomass) 328-341 (1988). Hydrocarbons are formed only in low yields.
Moreover, EP 0598229, U.S. Pat. No. 5,387,720 describe the acid-catalysed conversion of glycerol to acrolein in the gas phase and in the liquid phase. In this case, it is solely the acid strength (Hammett acid function) that determines suitability as a catalyst. DE 42 38 492 relates to the synthesis of 1,2- and 1,3-propanediol by dehydrating glycerol with high yields.
WO 2006/087083 discloses a process for preparing acrolein from glycerol over acidic catalysts, in which oxygen is added to the reaction mixture.
A similar process is described in WO 2006/087084. The catalysts used there have a Hammett acidity Ho in the range of −9 to −18.